Interrupts

Introduction:

Interrupt can stop job and start another more important job, and every part of information below support you to understand how interrupts work.

What is an interrupt?

Definition : an event external to the currently executing process that causes a change in the normal flow of instruction execution. Usually generated by hardware devices external to the CPU.

Why Interrupts?

 People like connecting devices .

•  A computer is much more than the CPU 

• Keyboard, mouse, screen, disk drives

• Scanner, printer, sound card, camera, etc.

 These devices occasionally need CPU service

• But we can’t predict when.

 External events typically occur on a macroscopic

 timescale .

• we want to keep the CPU busy between events

Alternative: Interrupts

Give each device a wire (interrupt line) that it can use to signal the processor 

• When interrupt signaled, processor executes a routine called an interrupt handler to deal with the interrupt.

• No overhead when no requests pending.

Intel 430HX Motherboard :

Programmable interrupt controller (PIC) part of the “South-bridge” chip :

• Commonly 8259A chip

•   8 inputs, 1 output

•   Can be chained together

• Newer systems use “Advanced PIC” (APIC) for SMP support Principle is the same

Hardware Interrupt Handling :

 1- Details are architecture dependent!

• Interrupt controller signals CPU that interrupt has occurred, passes interrupt number.

• Interrupts are assigned priorities to handle simultaneous interrupts.

• Lower priority interrupts may be disabled during service.

2- CPU senses (checks) interrupt request line after every instruction; if raised, then:

 

• Uses interrupt number to determine which handler to start.

• Interrupt vector associates handlers with interrupts.

3 - Basic program state saved (as for system call) .

4 - CPU jumps to interrupt handler .

5-  When interrupt done, program state reloaded and

program resumes .

Software Interrupt Handling :

Typically two parts to interrupt handling

• The part that has to be done immediately

• So that device can continue working

• The part that should be deferred for later

• So that we can respond to the device faster

• So that we have a more convenient execution context

This link is help you to more understanding about interrupt of operating system.

http://en.wikipedia.org/wiki/Interrupt

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